Belgian researchers tested the common--and perhaps common-sense--belief that runner's knee, or what medical professionals call patellofemoral pain, is caused by dysfunction in the quadriceps muscles. They had 46 people who reported pain from runner's knee and 30 people who didn't have runner's knee do a squat exercise. Their reasoning was that the squat exercise involves dynamic balance of the patella, or knee cap, so under the commonly held belief, the quads of those with runner's knee should act differently during the exercise.

But on the basis of MRIs taken before and after the squat exercise, there was no difference in how the two groups activated their quad muscles. That is, to do the squat, those with runner's knee recruited quad muscles similarly to how the pain-free people did. "As the relative contribution of the quadriceps muscles to a functional activity has not been modified, there is no evidence for quadriceps dysfunction," the researchers concluded.

This study supports the growing consensus that runner's knee (and many other common overuse injuries) stem not necessarily from weakness in the area immediately around the site of the injury, but from hip weakness and instability. In the case of runner's knee, it's thought that misalignment occurs in the knee because the hips aren't strong enough to control how quickly and how far the patella moves during the gait cycle.